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Epigenetic DNA tags Mostly, but not all, Removed in Embryos | Science News

2015-06-04

In the first weeks after conception, some of the cells in human embryos get their genetic blueprints scrubbed clean, conclude three new studies published June 4 in Cell. Those cells, the ones that become sperm or eggs, could beget the tiny embryos’ future offspring.

via DNA tags mostly deleted in human germ cells | Science News.

I previously wrote a post about epigenetic tags that are placed on DNA to alter the activity of genes– tags that record responses to environmental factors such as childhood abuse and differences in diet.  These tags appear to be related to a number of human diseases and characteristic responses.  For example, children raised in an extremely high stress environment grow up with an increased sensitivity to stress.  These children, as adults, have greater hormonal responses to stressful events: higher levels of cortisol and norepinephrine in response to stress, for example.

One of the common epigenetic markers is methylation; methylated DNA at the right location will shut down or enhance the activity of the associated gene.  Wikipedia describes methylation, occurring on cytosine or adenine nucleotides, as “suppressing the expression of endogenous retroviral genes and other harmful stretches of DNA…methylation is essential for normal development and is associated with…genomic imprinting, X-chromosome inactivation [in female cells], suppression of repetitive elements, and carcinogenesis.”

Now, as reported in these publications in Cell magazine, studies find that most, but not all, of these epigenetic tags are removed from germ cells (those that are destined to become sperm or egg cells) during embryo development.  It had been previously found in mouse embryos that virtually all epigenetic tags are erased from germ cells.  The new studies find that, in human germ cell embryonic precursors, about 4 percent of epigenetic tags are retained (as compared to 37 percent in another embryonic cell type.)  Some of the tags that are retained (“escapees”) relate to tendencies to schizophrenia, obesity, and multiple sclerosis, conditions that are known to run in families.   One of the Cell papers states: “Notably, many of the escapee-associated genes are expressed in brain, with potential links to neurological and metabolic disorders in humans.”

These studies show a significant difference between epigenetic inheritance in mice and humans.  Some things are epigenetically inherited in humans that are not in mice.  This opens the way for further study of epigenetics, a new area of research.

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